1. Field of the Invention
This invention relates to a displacement detector, and, more particularly, to an improvement of a displacement detector for detecting at an absolute position a relative displacement between first and second members by using periodic gratings.
2. Prior Art
There are two types of displacement detector used to measure a quantity such as the extent to which a tool of a machine tool is fed: an increment type of displacement detector which measures relative displacement between first and second members from the number of repetitions of a periodic primary signal formed by interaction between corresponding gratings relating to a track; and an absolute position type of displacement detector which detects at an absolute position a relative displacement between first and second members by using a plurality of primary signals formed by interaction between gratings endoded by, for example, largely changing pitches thereof with respect to each of tracks.
However, if, in the former, namely, the increment type of displacement detector, count errors occur, they are simply accumulated, directly resulting in an error in the measurement.
In the latter, namely, the absolute position type of displacement detector, occurrence of a count error does not result in an error in measurement since a displacement is measured at an absolute position. However, the gratings are fully encoded in such a manner that, as in a conventional method, the pitch of the grating in a second track is set to be twice as large as that of the grating in a first track; and the pitch of the grating in a third track is set to be twice as large as that of the grating in the second track . . . , occurrence of a detection error in one of the tracks or, in particular, one having a large pitch results in a large deviation of a measured value.
In addition, if an absolute position type of displacement detector is constituted by using optical gratings, it is necessary to set the same gaps in the gratings in each of the tracks in order to use a light source and an index scale in common. However, in the case where the encoded gratings are used, such equal gaps make it impossible to obtain good light reception signals because the difference between the pitches of the gratings in the different tracks is too large.